The present disclosure relates to an overload protection device, and particularly relates to an overload protection device applied to a thermal magnetic trip unit for a breaker.
As for the present thermal magnetic trip unit with less rated current (for example, 15 A, 16 A, 20 A etc.), the general problems thereof are lower temperature rising, minor deflection of a bimetallic strip, thus causing unreliable overload protection, that is, it is easy for a late release or a false release to occur. When manufacturing such trip units, they are usually subjected to difficulties of industrialized thermal tuning and a higher rework rate, thereby increasing the manufacturing cost. Furthermore, the massive short-circuit current readily causes damage to the bimetallic strip when it is flowing through the bimetallic strip.
For example, in the present directly-heated trip unit with lower rated current, the rise in temperature of the bimetallic strip in a current loop mainly depends on the heat generated by the bimetallic strip per se. However, such heat output is low due to the limited length of the bimetallic strip, and further, due to the fact that the bimetallic strip is connected to the client terminals directly through an electrically conductive braided wire so that heat dissipation is rapid. The bimetallic strip thus has a lower rise in temperature under a certain current and a minor deflection, its reliability for the overload protection is low and the thermal tuning is difficult. At the same time, the bimetallic strip is easy to be overheated and damaged under the short circuit.